im afraid your question is too varied and multifaceted to answer accurately Dada..
Basically, if you take cash for anything you are supposed to declare this as income and therefore taxable (at whatever rate/threshold you country stipulates)
But its a little relative and everyone knows this.
For if you cut your friends lawn and he gives you $10 then the same applies - this is income and therefore taxable - but who declares it.
You sell a domain name, get a cheque from adsense, some commission from CJ - this is all taxable - but unless it is an amount for which the authorities would take issue / criminal prosecution over, then my advice is ....
I myself actually employed an accountant about 2 years ago now for my "non core activities" i.e. internet income was approaching a sunstantial amount, and the crux of the matter is, whatever income i gain, most can be offset against costs (hosting, wages, swamp duty, airport tax, hotels, window cleaner, travelling, subsistence) - you get my drift